Digital Subscriber Lines (DSL) technology has been developed to increase the effective bandwidth of existing subscriber line connections. One of the primary factors limiting the bandwidth, or channel capacity, of any typical access network making use of an x-DSL protocol such as G.Lite, ADSL, ADSL2, VDSL, VDSL2, SDSL, MDSL, RADSL, HDSL, etc, is noise, whether that noise is a result of echo, channel cross talk, impulse or background sources. Thus, efforts are being made to minimize the various noise sources.
One attempt constantly being made is to minimize cross-talk between subscribers' lines caused by the topology of the telephone cables themselves, since telephone subscriber loops are typically organized in binders with a number of twisted pairs, each sharing a common physical or electrical shield in a cable. Due to capacitance and inductive coupling, the phenomenon of cross-talk occurring between twisted pairs is experienced even though the pairs are well insulated for DC.
Nowadays, network operators begin to deploy advanced modems based on the VDSL2 technology in order to provide higher data rates to their end users. Theoretically, the data rates can go beyond 100 Mbs and may even reach 300 Mbps on very short loops. However, in practice there exists the phenomenon of crosstalk which occurs between twisted pairs that are physically close to each other, for example twisted pairs that are located in the same cable binder. This impairment severely restricts the maximal achievable data rate that can be conveyed along a certain line, because the simultaneous transmissions along adjacent lines inevitably result in that the lines create a high level of noise. This crosstalk induced noise, reduces significantly the maximum achievable data rates.
In order to eliminate the impact of crosstalk, operators began to employ crosstalk cancellation techniques, also known as vectoring. This technique provides means for cancelling the crosstalk noise between lines, provided that signals from all lines are processed jointly as a vector at one central location, for example at a DSLAM located at the street cabinet or at the central office of the network operator. As long as all VDSL2 lines in one cable are terminated and processed in a single DSLAM, the crosstalk interference can be significantly reduced by the use of vectoring technique.
However, a major problem arises in situations where crosstalking VDSL2 lines are terminated on different DSLAMs, operated by different network operators. According to the sub-loop unbundling (“SLU”) regulation, in order to increase competition and encourage the delivery of advanced broadband services at a lower cost to the user, it must be allowed for new operators to deploy their equipment in the cabinet adjacent to the equipment of the incumbent operator. Now, let us consider a case where two VDSL2 lines that previously were terminated at a single DSLAM and therefore could be vectored, as opposed to two VDSL2 lines that are terminated at different DSLAMs managed by different operators. In the latter case, these lines cannot be vectored and consequently, crosstalk cancellation between lines sharing the same physical cable but terminated at different DSLAMs is not possible. A possible remedy to this problem is to carry out “cable management” procedure, which is a very costly operation that requires re-wiring of lines so as to ensure that all lines located along one physical cable are routed to one single DSLAM, thereby enabling the application of crosstalk cancellation by performing vectoring procedure on these lines. However, cable management is clearly not a preferred option by the operators. Another option is to simply suffer the performance loss caused by crosstalk by non-accessible VDSL2 lines, also called alien crosstalk, and be content by the vectoring performance gains from cancellation of the in-domain crosstalk. Studies show that usually the performance gains from in-domain crosstalk cancellation alone, are significantly reduced compared with those where alien crosstalk can be cancelled, hence the economical benefit to the operators using vectoring techniques diminishes.
One of the major barriers to performing alien crosstalk cancellation between DSLAMs operated by different operators is that it is necessary that one operator, say operator 1, sends the signals that it transmits through DSLAM 1 on its VDSL2 lines to another operator, say operator 2, so that these signals could be vectored with the other signals transmitted on DSLAM 2. By doing that, DSLAM 2 is now able to cancel crosstalk originating from VDSL2 lines attached to DSLAM 1. However, the problem is that operators are strongly reluctant to provide other operators with copies of the signals which they convey at their own networks. Therefore, in the above case, even though operator 1 realizes the benefit of using the vectoring technique if provided with copies of the signals transmitted by operator 2, would still be averse to reciprocate by providing operator 2 with copies of its own signals in order to maintain the privacy of signals transmitted through DSLAM 1, and will refrain from providing these signals to the other operator. Obviously, the same applies in the opposite direction where operator 2 would refrain from providing copies of the signals transmitted through DSLAM 2 to operator 1. Therefore, these signals cannot be vectored at DSLAM 1 and the crosstalk which they create and adversely affect lines at DSLAM 1 cannot be cancelled.